1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as transformers, and more specifically to electrical inductive apparatus which includes a liquid dielectric and coolant.
2. Description of the Prior Art
Since the early 1930's, electrical transformers for use in or adjacent to buildings, or other occupied areas, and on electric railroad cars, have been constructed with a polychlorinated biphenyl insulating and cooling liquid, which liquids are commonly called PCB's. The PCB's, which have a high dielectric strength, were chosen for these applications because of their fire-resistant characteristics.
The PCB's are toxic, i.e., they have the capacity to produce injury or illness to humans and animals through absorption, inhalation or ingestion, and strict procedures have always been observed in their handling, use and disposal. The PCB's, however, have a high chemical stability and despite strict handling and disposal procedures, they have appeared as a pollutant in areas of great industrial activity. Thus, the Federal Toxic Substances Control Act, passed in 1976, has made it mandatory that the use of PCB's in industry be phased out over a short period of time.
Silicone liquids, such as mixtures of the dimethyl siloxane polymers, have good electrical and fire-resistant characteristics, and they are non-toxic. Thus, the dimethyl-silicone liquids, such as Dow Corning's DC-561, are now being used in new electrical transformers for applications which formerly would use a PCB-filled transformer.
In certain instances, it would be desirable to retrofit existing PCB-filled transformers with an acceptable non-toxic, fire-resistant liquid, such as the silicone liquids. For example, when a PCB-filled transformer is repaired, the PCB liquid will be properly disposed of and the transformer filled with an acceptable substitute. Also, in certain applications, such as railway transformers, it may be desirable to replace the PCB's with an acceptable substitute on a scheduled basis, even though the transformers would not otherwise require repair.
The retrofitting of a PCB-filled transformer with an acceptable substitute liquid, however, quickly results in the contamination of the new liquid above the acceptable 500 part per million level. The PCB's have impregnated the solid cellulosic insulation (wood and paper), and other fibrous insulating materials used in the transformer, and the PCB's continuously leach out of the solid insulation into the originally acceptable liquid substitute. It is too costly to periodically check such retrofilled transformers for PCB pollution level, and to then replace the contaminated liquid when the PCB content exceeds 500 parts per million. Thus, while it would be desirable to retrofit a PCB-filled transformer by removing the PCB liquid and replacing it with an acceptable substitute, the PCB's and the new liquid must be economically maintained below 500 parts per million, before such retrofitting would be practical.